1. Field of the Invention
The present invention relates to a coupling arrangement for a welding apparatus intended in particular for MIG welding, the arrangement comprising a controllable direct-voltage source having a short reacting time.
2. Description of the Prior Art
In previously known welding apparatus which operate with direct voltage, the desired welding characteristics are obtained by means of suitably selected coupling in parallel of an inductance L and a resistance R, connected into the main current branch, as shown in accompanying FIG. 1. In this case, in a drop short-circuit situation the voltage U taken by the load changes as shown in FIG. 2a and the current I as shown in FIG. 2b. The change in current at the initial moment of the drop short-circuit is in this case .DELTA.I=E/R and the current increase rate respectively dI/dt=E/L.
The welding characteristics of a welding apparatus are greatly dependent on these quantities, and various resistor-choke combinations must be connected to the circuit in order to adjust the characteristics to different situations. Owing to the high currents, the adjustment is difficult, and it is necessary to use large and expensive components. The quantity E is normally adjusted by means of a coil switch located on the primary side of the transformer, since phase control is not as such applicable to voltage control, with the exception of very narrow-range vernier control. The reason for this is the strong ripple caused by phase control, and for dynamic reasons this ripple cannot be eliminated by means of chokes.
It has turned out that by measuring the current and voltage of the apparatus and by electronically controlling, on the basis of the measured current and voltage, the direct-voltage source having a sufficiently short reacting time, the behavior illustrated in FIGS. 2a and 2b can be obtained without the described coupling in parallel of a resistor and a choke.
The present invention, which is based on this idea, provides a coupling arrangement of the character once described, which comprises a coupling arrangement for a welding apparatus intended in particular for MIG welding, the arrangement comprising:
a controllable voltage source having a short reacting time; a first set of circuit elements for detecting the current and voltage taken by the load and for forming voltage signals corresponding to these quantities;
a second set of circuit elements for forming a control signal corresponding to the said voltage signals so that in a drop short-circuit case, i.e. when the voltage suddenly drops to zero, the control signal corresponds to the values of the desired current step and the current increase rate; and a third set of circuit elements, which receive the control signal and which have been fitted to control the voltage source according to the control signal.
Thus, it is an object of the present invention to eliminate the above difficulties and to provide an electrically controllable source of welding current, in which the desired dynamic and static characteristics are obtained by means of electronic circuits and by applying the control technique approach.